IIRC, there is a fountain that will restore your age, though it's in a difficult location, monster-wise. I have all the strategy guides, I think, and I can look this up for you if you don't get an answer before then. I loved hiring the various npcs to travel with the party. I'd always end up with ghost towns until the NPC's respawn
If there isn't a fountain, I know that one of the black potions cures aging. I think it's called restoration or rejuvenation. Your best bet to find black potions are probably in the Celestial City or Bracada Desert at the magic shops. Of course, if you have a grandmaster alchemist you can make it yourself. Mix a Divine Restoration (white) with one of the two color potions, can't remember which…red/orange maybe? If red/orange is haste, I think that's one that mixes. Save in case you blow up
Magic shops are random, so save before entering. If you don't like the selection, restore your save and try again.
Red/Green Divine Restoration
Orange/Blue Divine Restoration
Yellow/Purple Divine Restoration